* mime/base64.c: the output buffer for the decoding was always one byte too small to keep the terminating NUL byte. This always went unnoticed even when running tools like Wipeout/Mungwall if the length of the source string was not a multiple of 8. In this case the granularity of AmigaOS' memory system shadowed this buffer overrun. Only for source strings with a length being an exact multiple of 8 bytes the buffer overrun eventually really happend. This closes #449.

* Config.c, mui/ConfigWindow.c, misc: separated data structure and GUI of the configuration as much as possible. This finally makes the old YAM_CO*.c files obsolete and future modifications to the config GUI should no longer trigger an almost complete rebuild of all modules using the config.

* YAM_RE.c, mime/uucode.c, mime/base64.c, mime/qprintable.c: modified mail
file decoding routines to always decode to UTF8 instead to the local
charset. This results in the temporary mail files to be always encoded
in UTF8 and then recoded to the local charset only when loading the text
in a texteditor object. This refs #7.

* YAM_COg.c, YAM_CO.c, misc: renamed "readCharset" variable to "localCodeset"
to make its purpose more clear. Also moved its GUI config items to the
"FirstSteps" config page and moved the external editor setup options to
"Misc". Added a possibility to specify a codeset to which text should
be automatically converted to when starting an external editor. If not
set the currently configured codeset of the write window is used or the
global one set in YAMs' configuration. This should address one item of
a long standing ticket and thus refs #7.

* YAM_RE.c, mime/base64.c, mime/qprintable.c, mime/uucode.c: having the option "automatic cyrillic charset detection" active even without the necessitiy to have to use cyrillic characters will no longer screw up encoded non-text attachments. This closes #349.

* base64.c: fixed bug in base64decode() where an incorrect pointer was used for
the calculation of the final output string length. This caused crashes when
extracting attachments from base64 encoded emails after the recent base64
API changes.

* mime/base64.c, misc: reworked the base64 encoding/decoding functions to
allocate the output string rather than relyig that the user supplies a
large enough string. This should eliminate potential buffer overruns and
make the whole API a bit more clean.

* misc: first version of sending mails via SMTP in a subthread. The transfer
progress will be shown as usual, but for the future each transfer will have
its own separate graphical display. The window will be closed as soon as the
last transfer has finished.